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ISO 28199-2:2021 — Colour Stability, Process Hiding Power, Re-dissolving, Wetting, Surface Texture and Mottling for Spray-Applied Coatings
Paints and varnishes — Evaluation of coating properties related to spray application — Part 2
Introduction to ISO 28199-2:2021
ISO 28199-2:2021 is part of a multi-part standard developed by ISO/TC 35 (Paints and varnishes) in collaboration with CEN/TC 139. It specifies test methods for evaluating coating materials applied via spray application, focusing on seven critical properties: colour stability, process hiding power, re-dissolving, overspray absorption, wetting, surface texture, and mottling. These methods are based on studies by the European Council for Automotive R&D (EUCAR) and are widely adopted in automotive OEM coating lines and industrial paint shops.
For coating process engineers, these seven parameters form the core diagnostic toolkit for troubleshooting spray application defects. Mastering ISO 28199-2 enables rapid identification of root causes in colour variation and surface quality issues.
Colour Stability and Process Hiding Power
Colour stability is assessed by measuring colour space values (L*, a*, b*, C*, h) across a wedge-shaped coating of increasing film thickness. A stable colour is achieved when the colour curve becomes approximately parallel to the film thickness axis. The range between tmin (start of stability) and tmax (end of stability) defines the process window for colour-consistent application. Process hiding power extends this concept by evaluating the minimum film thickness at which the substrate is fully covered — a critical parameter for both quality and material cost optimization.
| Property | Evaluation Method | Key Parameter | Typical Automotive Target |
|---|---|---|---|
| Colour stability | L* vs film thickness plot | tmin to tmax range | Plateau length ≥ 15 μm |
| Process hiding power | Substrate coverage point | tmin (hiding) | ≤ 20 μm for solid colours |
| Re-dissolving | Clear coat wedge on base coat | Drift onset thickness | ≥ 40 μm clear film build |
| Overspray absorption | Two-stage spray L* clustering | Transition smoothness | No distinct L* clusters |
| Surface texture | Short/long wavelength values | Mean within process window | Short-wave < 15, Long-wave < 10 |
| Mottling | L* variation across spray passes | Cluster separation | ΔL* < 1.0 between passes |
Re-dissolving, Overspray Absorption and Wetting
Re-dissolving occurs when solvents from a subsequently applied clear coat partially re-dissolve the base coat, disturbing effect pigment orientation. This manifests as a drift in colour values at increasing clear-coat film thickness. Overspray absorption evaluates how well the base coat absorbs overspray from a second application pass — poor absorption leads to visible lightness clusters. Wetting behaviour is determined by measuring long-wavelength (3 mm) surface texture values as a function of clear-coat film thickness, identifying the point at which the coating first wets the substrate completely.
In multi-layer metallic paint systems, re-dissolving is the most common cause of “mottled” appearance in production. Always verify the clear-coat solvent formulation compatibility with the base coat binder system before production ramp-up.
Surface Texture and Mottling Evaluation
Surface texture is characterized through short-wavelength (<3 mm) and long-wavelength (>=3 mm) components measured via profilometry. Short-wavelength values primarily reflect base coat quality, while long-wavelength values relate to clear coat levelling. The 2021 revision introduced specific limit values for these parameters. Mottling is evaluated through both colour measurement (L* clustering) and visual assessment under agreed lighting conditions. The mottling range is typically bounded by the hiding power limit on the low end and the re-dissolving onset on the high end — making it a crucial process window parameter.
A well-designed process window for base coat film thickness should simultaneously satisfy colour stability, hiding power, and mottling-free application. Modern automotive paint lines typically target 12-18 μm for base coat dry film thickness.
Engineering Design Insights
Integrating ISO 28199-2 test methods into production quality control enables paint shops to establish robust process windows, reduce material consumption through optimized hiding power, and systematically troubleshoot colour-related defects. The standard is particularly valuable for transitioning between coating material suppliers or introducing new colour formulations, as it provides objective pass/fail criteria for spray process compatibility.
Q1: What is the minimum film thickness for reliable colour stability testing?
A: The test panel should be prepared with a wedge-shaped coating covering 0-50 μm for base coats, with colour measurements taken at intervals of 2-5 μm across the wedge.
A: The test panel should be prepared with a wedge-shaped coating covering 0-50 μm for base coats, with colour measurements taken at intervals of 2-5 μm across the wedge.
Q2: Can ISO 28199-2 be used for waterborne paint systems?
A: Yes, the standard is applicable to all spray-applied coating systems including solvent-borne, waterborne, and high-solids formulations, provided the test panel preparation follows ISO 28199-1.
A: Yes, the standard is applicable to all spray-applied coating systems including solvent-borne, waterborne, and high-solids formulations, provided the test panel preparation follows ISO 28199-1.
Q3: How does mottling differ from colour instability?
A: Mottling specifically refers to L* variation between sequential spray passes at the same film thickness, whereas colour instability describes colour change across different film thicknesses within a single application.
A: Mottling specifically refers to L* variation between sequential spray passes at the same film thickness, whereas colour instability describes colour change across different film thicknesses within a single application.
Q4: What equipment is needed for surface texture measurement per this standard?
A: A contact or non-contact profilometer capable of measuring short-wavelength (<3 mm) and long-wavelength (>=3 mm) components, typically with a measurement range of 0.1-100 μm vertical resolution.
A: A contact or non-contact profilometer capable of measuring short-wavelength (<3 mm) and long-wavelength (>=3 mm) components, typically with a measurement range of 0.1-100 μm vertical resolution.
